The use of conjugated polymers in electroluminescent polymer diodes has been demonstrated by the group of prof. Richard Friend in Cambridge (J. H. Burroughes, D. D. C. Bradley, A. R. Brown, R. N. Marks, K. Mackay, R. H. Friend, P. L. Burns, and A. B. Holmes, Light-emitting diodes based on conjugated polymers, Nature, 347(6293), (1990) 539-41.) and received intense interest from a wide range of scientific and industrial groups in the U.S.;
(D. Braun and A. J. Heeger, Visible light emission from semiconducting polymer diodes, Appl. Phys. Lett., 58(18), (1991) 1982-4, D. Braun, A. J. Heeger, and H. Kroemer, Improved efficiency in semiconducting polymer light-emitting diodes, Journal of Electronic Materials, 20(11), (1991) 945-8.), PA1 (G. Grem, G. Leditzky, B. Ullrich, and G. Leising, Realization of a blue-light-emitting device using poly(p-phenylene), Advanced Materials, 4(1), (1992) 36-7.G. Grem and G. Leising, Electroluminescence of wide-bandgap chemically tunable cyclic conjugated polymers, Synt. Metals, 57(1), (1993) 4105-4110.)
Austria:
Germany: (S. Karg, W. Riess, M. Meier, and M. Schwoerer, Characterization of light-emitting-diodes and solar-cells based on poly-phenylene-vinylene, Synt. Metals, 57(1), (1993) 4186-4191.)
and Japan: (Y. Ohmori, M. Uchida, K. Muro, and K. Yoshino, Effects of alkyl chain length and carrier confinement layer on characteristics of poly(3-alkylthiophene) electroluminescent diodes, Solid State Commun., 80(8), (1991) 605-8, Y. Ohmori, M. Uchida, K. Muro, and K. Yoshino, Visible-light electroluminescent diodes utilizing poly(3-alkylthiophene), Japanese Journal of Applied Physics, Part 2, 30(11B), (1991) L1938-40., Y. Ohmori, M. Uchida, K. Muro, and K. Yoshino, Blue electroluminescent diodes utilizing poly(alkylfluorene), Japanese Journal of Applied Physics, Part, 30(11B), (1991) L1941-3.
These devices are prepared by sandwiching a thin film (0.05-1 .mu.m) of a conjugated polymer in between two electrode materials. The polymer is characterized by a bandgap between approximately 1.5-4 eV, and the cathode injects electrons into the lowest unoccupied molecular orbitals (LUMO) of the polymers, while the cathode injects holes into the highest occupied molecular orbitals (HOMO) of the polymer. These opposite charges meet, recombine and electroluminescence is observed in the form of photon emission. Details in these processes are described in available literature ((R. H. Friend, J. H. Burroughes, and D. D. C. Bradley, Electroluminescent Devices, Patent Application PCT/GB90/00584, (1990) )(I.Parker: Carrier Injection and Device Characteristics in Polymer Light Emitting diodes, J.Appl.Phys 75 (1994) 1666.)).
A problem that has not yet received an effective solution is that of obtaining different colours from one individual polymer LED.
Many materials have today been shown to give electroluminescence in polymer LEDs, among which are found the main groups of (substituted) poly(paraphenylene vinylene) and (substituted) polyparaphenylenes. Using different substituents on these polymers, it is possible to obtain light with colours all the way from blue to green, yellow, orange and red. The colour of the polymer LED is given from the chemical and electronic structure of the polymer, and cannot be changed except by changing the polymer in the LED. Among these main chain polymers, with the various substituents, there is no one which covers the full range of the visible spectrum. There is therefore no way of forming a full colour polymer LED using these materials.
Another means of forming a polymer LED is to use a nonconjugated prepolymer and to convert this into different forms, capable of giving emission at different wavelength. The conversion occurs by thermal or chemical methods, or combinations thereof. It is therefore possible to pattern the prepolymer in such a way as to prepare pixels of different colours adjacent to each other. (P. L. Burn, A. B. Holmes, A. Kraft, D. D. C. Bradley, A. R. Brown, and R. H. Friend, Synthesis of a segmented conjugated polymer chain giving a blue-shifted electroluminescence and improved efficiency., J. Chem. Soc., Chem. Commun., (1992) 32., P. L. Burn, A. B. Holmes, A. Kraft, D. D. C. Bradley, A. R. Brown, R. H. Friend, and R. W. Gymer, Chemical tuning of electroluminescent copolymers to improve emission efficiencies and allow patterning, Nature, 356(6364), (1992)47-9.)
So far, there has not been demonstrated a prepolymer that can be converted to different conjugated forms which will give colours all over the full visible spectrum. The limited wavelength range attainable is a serious limitation; there is no possibility of forming red, blue and green pixels close to each other. The thermal/chemical conversion also affects the structure of the polymer layer, and residual chemicals, or vacancies, may exist in the converted polymer film. This may seriously affect the electrical and mechanical stability of the layer.
It has also been demonstrated how multilayers of conjugated polymers can be prepared by putting thin films on top of each other by spin coating. If these different layers have different colours of emission , it has been demonstrated that the resulting colour emission from the multilayer can be influenced by the applied voltage, by shifting the emission process between the different polymer layers. Some degree of colour control is attainable by this method, but this is a very limited freedom, as the different colours all fall in a narrow wavelength range. (A. R. Brown, N. C. Greenham, J. H. Burroughes, D. D. C. Bradley, R. H. Friend, P. L. Burn, A. Kraft, and A. B. Holmes, Electroluminescence from multilayer conjugated polymer devices: spatial control of exciton formation and emission, Chem. Phys. Lett., 200, (1992) 1-2.)
A very different means of obtaining colour control of the electroluminescence from thin organic layers has been presented by workers at Bell Labs, and others. Here a wide band emitter--normally a molecular compound--is positioned above an optical microcavity. The geometry of the cavity is chosen so as to give enhanced emission modes at red, green and blue wavelengths, respectively. In this way it is possible to generate pure elemental colours by patterning a surface with optical microcavities of different resonance wavelengths, and thus making adjacent red, green and blue pixels for obtaining a full colour screen.
(Dodabalapur-A Rothberg-L J Miller-T M "Electroluminescence from Organic Semiconductors in Patterned Microcavities" Electronics Letters Vol 30 Iss 12 pp 1000-1002 1994 (NV160)
Dodabalapur-A Rothberg-L J Miller-T M Kwock-E W: "Microcavity Effects in Organic Semiconductors" Applied Physics Letters Vol 64 Iss 19 pp 2486-2488 1994
Dodabalapur-A Rothberg-L J Miller-T M: "Color Variation with Electroluminescent Organic Semiconductors in Multimode Resonant Cavities" Applied Physics Letters Vol 65 Iss 18 pp 2308-2310 1994)
A rather different means of obtaining variable colours from polymer light emitting diodes is also reported (M. Uchida, Y. Ohmori, T. Noguchi, T. Ohnishi, and K. Yoshino, Color-variable light-emitting diode utilizing conducting polymer containing fluorescent dye, Japanese Journal of Applied Physics Part, 32, (1993)). The colour of the emitted light can be controlled by choosing different voltages to drive the diode. The diode is built from a conjugated polymer layer within which a conjugated molecule is dispersed. By choosing different forward voltages, smaller or larger emission from the conjugated molecule is observed. This is reported to occur at 70 K, during measurements inside a cryostat. The mechanism behind this phenomenon is not known. Such polymer/molecule combinations will however be rather unstable, as the molecule tends to crystallize inside the polymer matrix in the absence of strong attractive interactions between the molecule and polymer.